We have now dealt with a number of incidents where people have become lost using a mobile phone to navigate. These devices may well work if the right mapping is installed, and the instructions have been fully understood, but otherwise risk being worse than useless. They also drain batteries significantly faster than when using the phone only.. BEWARE!! Car sat-navs are equally useless.. The less preparation you make, the more likely you are to spend a cold, miserable, wet night out on the fells!